Area Forecast Discussion
Issued by NWS Cheyenne, WY

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664
FXUS65 KCYS 060335
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
835 PM MST Mon Dec 5 2016

.UPDATE...
Issued at 820 PM MST Mon Dec 5 2016

The Winter Weather Advisory and Wind Advisory have been allowed to
expire as scheduled this evening. Mountain show showers could very
well persist through the night, but believe impacts are minimal w/
additional accumulations only a few tenths of an inch. Gusty winds
persist over the southern Panhandle, but continue to decrease with
gusts generally around 30-35 MPH. Snow showers will end across the
high plains, but webcams continue to show slick/snow covered roads
so will highlight potential travel hazards in our Social Media and
HWO products. A very cold night is still on track w/ temps already
in the teens and upper single digits, especially along and west of
the Laramie Range.

Accumulating snow will likely return on Tuesday. The mountains are
expected to start first during the mid to late afternoon, and then
spreading into the I80 corridor from LAR-SNY and areas as far N as
BFF during the evening and overnight. We gave consideration to the
issuance of a new Winter Weather Advisory for the Snowy and Sierra
Madre ranges, but there are concerns with a quick arrival of cold/
dry arctic air which may limit mountain accumulations. This may be
the case over the plains as well, so decided to let the next shift
take another look before issuing any headlines.

&&

.SHORT TERM...(This evening through Tuesday night)
Issued at 240 PM MST Mon Dec 5 2016

Strong cold front has pushed through the whole forecast area early
this afternoon and is now in central Nebraska and northern
Colorado. As expected...temperatures have dropped quickly after
fropa with temperatures already in the mid teens to early 20`s. An
area of locally heavy snow showers is now located across central
Albany county north of Laramie...and extends east into Platte and
Goshen county. This band has brought a quick half inch to one inch
of snow along with strong gusty winds. This band will continue to
move south into the I80 corridor over the next few hours...and
seems to be associated with the bulk of the colder air as
temperatures have fallen into the upper teens to low 20`s behind
the band with northerly winds. Will continue with the Winter
Weather Advisories for the mountains and Converse county into
this evening. Snow continues near Douglas and Glenrock...but has
ended for the most part across the Arlington area. Cold air will
settle across the area tonight with lows in the single digits east
of I25...and between zero to 10 below zero west of I25. Wind chill
values are already pushing 10 below zero across portions of the
area with gusty winds...believe these values will range between 10
below to 25 below zero along and west of I25 tonight and early
Tuesday morning.

For Tuesday models are in better agreement with the second cold
front...which will usher in an arctic airmass. All models now
show an upper level shortwave digging southeast into the eastern
Great Basin region on Tuesday with ample jet energy/dynamics
aloft. Still expect snow over the southern half of the forecast
area...especially along I80. However...the lack of warm air
advection aloft and the main shortwave being several hours later
than previously expected may result in limited moisture advection
with the earlier arrival of the arctic airmass. In fact...most
locations north of the Platte river valley may receive little if
any snowfall at all as the arctic air moves in early. With very
cold air across the area...snow ratios should be high.
However...the dendritic growth zone is a little too cold for
rapid snow accumulation...although winds should be light enough
aloft. Do believe it is good enough for a 6 hour period late
Tuesday afternoon into tonight...so still expect areas along and
east of the southern and central Laramie Range to see 2 to 5
inches of snow with locally higher amounts. The mountains
including the I80 summit will see a bit more...and generally
between 4 to 8 inches. With all the active weather today...decided
to let the evening and midnight shift take a closer look at it for
potential headlines. Do expect at least Winter Weather Advisory
conditions for snow from east of Rawlins eastward to the southern
Nebraska panhandle...including all of the I80 corridor through
Sidney and Kimball Nebraska. Thankfully...winds should not be much
of an issue with this system with north to northeast winds
generally between 10 to 20 mph through Tuesday night. This may
result in some drifting snow across area roadways but blowing snow
is not expected. Daytime highs on Tuesday will be cold and in the
teens to middle 20s...which will seem warm in comparison to
Tuesday night and Wednesday. The arctic front will arrive Tuesday
night...lowering temperatures between zero to 10 below zero (SE
Wyoming) and a few degrees above zero (western Nebraska). Frigid
wind chills between 10 below to 25 below zero are also possible.
Expect steady light snow most of the night southeast of a Laramie
to Torrington line.

.LONG TERM...(Wednesday through Monday)
Issued at 230 PM MST Mon Dec 5 2016

Some light snow or flurries may still be ongoing over the southern
parts of the CWA Weds morning but this is expected to end by late
morning. Very cold temperatures will be over the CWA Weds and Weds
night as a cold arctic high settles down the high plains. Maximum
temps only in the single digits to low teens Weds with lows
Thursday morning from -5 to -20 over the CWA as winds diminish and
skies clear. A little warmer Thursday but still quite cold. Temps
then tricky Thursday night as the surface front begins to retreat
eastward so mins may be attained in the evening with rising
temperatures during the rest of the night as surface winds turn
downslope and increase some. Much warmer Friday and continued
relatively mild Saturday as the CWA will be under westerly upper
flow. A shortwave passing by late Saturday should drive a quick
shot of colder air down the high plains Saturday night but pulling
back out Sunday night. Milder air returns for Monday. Mainly dry
over lower elevations through the period but the higher mountains
look like they will have periods of snow showers for the entire
period given favorable moist westerly flow riding up the slopes.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 458 PM MST Mon Dec 5 2016

Mainly VFR conditions are expected this evening and overnight with
snow showers continuing to decrease in coverage. Winds will remain
gusty through much of the evening. Expect deteriorating conditions
after 21z Tuesday, especially along the I-80 corridor between KLAR
and KSNY as accumulating snow returns to the region.


&&

.FIRE WEATHER...
Issued at 145 PM MST Mon Dec 5 2016

No Fire Weather concerns this week as periods of accumulating
snow and bitter cold temperatures are expected. Relative
humidity will remain high with excellent overnight recoveries.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...CLH
SHORT TERM...TJT
LONG TERM...RE
AVIATION...CLH
FIRE WEATHER...TJT



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